Surf the latest with our weekly newsletter.

Why Beaches Sometimes Stink—And Why It Might Mean Danger

Seaweed washing up on coastlines usually just makes an bad beach smell. But when certain types of seaweed collect in piles, it can slowly form a toxic gas.

Let’s be honest: sometimes being by the beach is stinky. There’s the putrid smell of low tide, the fishy smell of live bait, and sometimes there’s something else—a rancid odor that can signal danger.

Seaweed washing up on coastlines is usually just an annoyance. But when certain types of seaweed collect in piles, still wet and baking in the sun, it slowly decomposes and a gas can form below a visible white crust.

Why? The anaerobic breakdown of sulfates in the seaweed leads to the production of hydrogen sulfide gas. Then, when the white crust on the seaweed breaks because of waves rolling ashore or people stepping on the seaweed, the gas releases into the air and gives off that all-too-familiar rotting eggs smell.

In low concentrations and in open spaces, the gas is fairly harmless—just awful to smell. It’s actually naturally produced and broken down in the body, so we can tolerate it indefinitely in low quantities. The problems begin when large amounts of seaweed pile up on beaches and produce high levels of the gas.

Hydrogen sulfide is a toxic gas, and in high concentrations and/or enclosed places it can cause fatigue, headaches, and sore throats—even vomiting. (Not exactly what you're hoping for on a beach vacation.) The most sensitive are those who have respiratory problems, the elderly, children, and pregnant women.

That sounds pretty frightening, but there's no need to swear off the beach just yet.

Cases like the one in France are rare. But global warming is contributing to the problem, as warmer winters allow the seaweed to live through the winter which in turn creates more build-up on the beaches. Plus, reports speculate that especially in the case of France's beaches, nitrogen dumped into water as pollution also seriously contributes to the problem.

Some communities are dealing with decomposing seaweed in a number of ways: some will close beaches if it gets too bad, others have formed community committees to monitor and remove the seaweed.

Most importantly, though, we can all do our part to lower our own carbon emissions.